a) Field of the Invention
The present invention is directed to an optical system for an illumination device of a microscope.
b) Description of the Related Art
To examine a specimen with a microscope, the specimen is usually illuminated by means of an illumination arrangement. In transmitted illumination, the light of the illumination arrangement is radiated through the specimen, whereas in incident illumination the illumination arrangement radiates light on the surface of the specimen that is examined by means of the microscope. Different requirements are imposed on the illumination arrangement for different illumination situations in incident light studies, for example, illumination for fluorescence microscopy at different wavelengths, darkfield illumination, white-light incident illumination, or structured incident illumination at different wavelengths.
In view of the different requirements for incident illumination, the construction shown schematically in FIG. 1 has proven useful. An illumination unit 1 with a light source 2 radiates a bundle of light rays on an optical system 4 with a collector 3 that images the light source 2 to infinity. First partial optics 5 generate an intermediate image of the light source 2 in an intermediate image plane and the intermediate image is imaged to infinity by second partial optics 6. The bundle of light rays exiting from the second partial optics 6 is deflected by a tube lens 7 and a semitransparent mirror 8 to the objective 9 and, after passing through the objective 9, illuminates an object 10. In conformity to the basic rules of Köhler illumination, an aperture diaphragm 11 is arranged in the intermediate image plane between the partial optics 5 and 6. The second partial optics 6 and the tube lens 7 image the intermediate image of the light source 2 in the objective pupil 12. This construction makes it possible to place a field diaphragm 13 in a plane in which a sharp intermediate image of the objective image is formed, namely, in the focal plane of the tube lens 7. Since a diffraction-limited intermediate image of the objective image is present in this plane, structures can be arranged which are then imaged with absolute sharpness on the object.
As a rule, neither the collector nor the partial optics are achromatic for reasons of cost. This leads to severe longitudinal chromatic aberrations of the light source image in the objective pupil. Particularly in illumination for fluorescence studies, these longitudinal chromatic aberrations cause a highly inhomogeneous illumination of the object field. This can only be avoided by readjusting the light source every time a filter is changed. This process is very cumbersome and makes the use of fast filter changers practically impossible.
This disadvantage can be overcome by using special collectors which are achromatic at least to the extent that the longitudinal chromatic aberration in the objective pupil is tolerable. However, such collectors have a very complicated construction.
Further, an intermediate image plane of the object image lies between the partial optics and the collector. Impurities in this intermediate image plane are imaged sharply on the object and considerably impede observation of the object. Elements of the achromatic collectors inevitably approach very close to this intermediate image plane, which leads to considerable problems with respect to the cleanliness of the collector lenses.